Qubit

Every normal computer stores information in tiny switches. Such a switch knows exactly two states: on or off, written as 1 or 0. It is called a bit. A qubit is the counterpart to this in a quantum computer, that is, in a machine that exploits the laws of physics governing very small particles. The crucial difference: a qubit does not have to decide between 0 and 1 as long as no one looks. It can be a proportion of both at once, for example 70 percent 0 and 30 percent 1.

Why eight qubits are more than eight bits

The benefit arises from the interplay of many qubits. Eight normal bits store exactly one of 256 possible numbers. Eight qubits, by contrast, can hold all 256 possibilities at once within themselves, each with its own share. Add one more qubit, and this number doubles. With 300 qubits, there would be more combinations than there are atoms in the visible universe.

However, this does not result in a miracle machine for everything. You cannot simply read out the many possibilities. Upon measurement, the state collapses, and you obtain only a single result of 0 or 1. A quantum program must therefore be constructed so that the wrong possibilities cancel each other out and the correct one remains. This only succeeds for certain types of tasks.

Two applications are considered particularly important. First, breaking today’s encryption, which many banks and messaging services use. Second, the simulation of molecules, for example for new medicines or better batteries. For word processing, video games, or training today’s AI models, however, a quantum computer offers nothing. Anyone who claims otherwise is usually selling something.

From atom to computational step

A qubit is not a component from an electronics store, but a single physical object. Often these are small superconducting circuits cooled to almost minus 273 degrees Celsius. Other companies use individual trapped atoms or particles of light. Computation happens by deliberately nudging these objects with microwave or laser pulses.

Entanglement is especially important here. In this process, two qubits are coupled so that their states become linked. Measuring one instantly determines the result of the other as well. It is only through such couplings that calculations arise which a classical computer cannot replicate.

The biggest problem is called decoherence. Qubits are extremely sensitive to heat, vibration, and radiation. Within mere fractions of a second, their state breaks down, and the calculation becomes useless. As a countermeasure, many physical qubits are bundled into a single stable logical qubit that corrects errors itself. Depending on the method, a logical qubit may require hundreds or thousands of physical qubits — which is why the raw qubit count of a machine says little about its actual performance.

Reading qubit numbers in headlines correctly

In the news, the term usually appears as a record announcement: IBM, Google, or a start-up unveils a chip with a new qubit count. Current systems range from a few hundred to over a thousand physical qubits. Error-corrected logical qubits, on the other hand, currently exist only in a handful of cases. When reading such reports, it’s worth asking which kind of qubit is actually meant.

A second key term is post-quantum cryptography. Because future quantum computers could break today’s encryption, authorities and corporations are already switching to new methods. The reason is simple: data intercepted today could be decrypted in ten years. This transition indirectly affects everyone who does online banking.

You can already try this out without your own lab. Several providers offer real quantum chips over the internet, sometimes for free for schools and students. You write a small program, send it to the data center, and get the measurement results back. An actual quantum computer in your living room remains unrealistic, however, if only because of the cooling technology required.